The present invent ion relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image transferring device for an image forming apparatus of the type using a belt formed of a dielectric material.
An image transferring device of the type described includes an endless belt formed of a dielectric material and a charging member for charging the belt. While the charging member charges the belt, a drive source causes the belt to run so as to convey a paper sheet or similar recording medium. In this condition, a toner image formed on an image carrier is transferred to the paper sheet. The belt conveys the paper sheet while electrostatically attracting it. A guide member guides the paper sheet separated from the belt toward a fixing unit.
A problem with the above-described configuration is that when the paper sheet carrying the toner image is separated from the belt, potential sharply rises due to a change in the gap between the charged paper sheet and the belt, resulting in peel discharge. The peel discharge disturbs the toner image, which is not fixed, and causes the toner to fly about and render the toner image defective. Discharge also occurs when the paper sheet is being conveyed along the guide member and when the former is separated from the latter. As a result, the toner contaminates the guide member and deposits on the rear or the edge portions of the next paper sheet by way of the guide member.
In light of the above, Japanese Patent Laid-Open Publication No. 6-317990 (Prior Art 1), for example, proposes an arrangement for absorbing the slack of a paper sheet ascribable to a difference in linear velocity between a fixing unit and a belt. In the proposed arrangement, a guide member has a guide surface thereof formed of a conductive material while a discharge brush is positioned at the outlet of the guide member for efficiently discharging a recording medium.
Japanese Patent Laid-Open Publication No. 6-180538 (Prior Art 2) discloses an image transferring device configured to reduce a change in the volume resistivity of the belt and a change in the volume resistivity of a guide member, thereby reducing the variation of an electric field. A plurality of ribs extend on the guide surface of the guide member in a direction in which the guide member guides a paper sheet (direction of guide hereinafter). Even when toner smears the guide member, the ribs prevent the paper sheet from directly contacting the smeared part of the guide member.
Prior Art 1 has a problem that the discharge brush excessively discharges a paper sheet at the outlet of the guide member and thereby obstructs electric attraction expected to act on the paper sheet at a fixation inlet guide member following the above guide member. Consequently, when the leading edge of the paper sheet enters the nip of a fixing unit while the trailing edge of the same is conveyed by the belt, the paper sheet is apt to warp at the position of the discharge brush due to a difference in linear velocity between the fixing unit and the belt. This brings about a defective image ascribable to discharge. Prior Art 2 exhibits a discharging effect not as great as one achievable with, e.g., a discharge brush and causes defects to appear on the toner image at the pitch of the ribs when the paper sheet is separated from the guide member.
As stated above, in the conventional image transferring devices, the discharge disturbs paper transfer if excessive or makes a toner image defective due to discharge if short.
To solve the above problem, Japanese Patent Application No. 11-254370 (Prior Art 3), for example, teaches an image transferring device including a guide member and a discharging member positioned at the intermediate portion of the guide member in the direction of guide. Specifically, the guide member is formed with holes open at its guide surface. The discharging member is received in the holes. With this configuration, the device is capable of stably conveying a paper sheet while causing the guide member electrostatically attracting the paper sheet. At the same time, the paper sheet is discharged and therefore suffers from a minimum of image deterioration ascribable to discharge that occurs at a position following the guide member.
However, Prior Art 3 has a drawback that the edges of the holes open at the guide or conveyance surface of the guide member are apt to catch the leading edge of a paper sheet, disturbing the conveyance of the paper sheet or causing the paper sheet to jam the path. Further, when the leading edge of the paper sheet caught by the edges of the holes rebounds, the resulting shock is likely to disturb and deteriorate a toner image carried on the paper sheet. Moreover, the holes, receiving the discharging member therein, lowers the mechanical strength of the guide member.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-13439, 7-261562, 9-218623 and 11-84903.
It is therefore an object of the present invention to provide an image transferring device capable of insuring the conveyance of a paper sheet despite the presence of holes, obviating defective images ascribable to discharge to occur at a position following a guide member, and preventing the mechanical strength of the guide member from decreasing.
An image transferring device of the present invention includes a belt for conveying a recording medium, which carries an image transferred thereto from an image carrier, while electrostatically attracting the medium. A charging device applies a charge to the belt. A belt drive source causes the belt to run. A guide member guides the recording medium separated from the belt. A plurality of holes are formed in the intermediate portion of the guide member in a direction of guide and open at the guide surface of the guide member. A discharging member is received in the plurality of holes. A plurality of ribs extend on the guide surface of the guide member in the direction of guide in such a manner as to bridge the edges of adjoining ones of the plurality of holes.